The ruling was a rare setback in Lagarde's glittering career but it was not clear how it would affect her position at the International Monetary Fund, whose board was to meet in Washington to discuss the court's decision.
A special court in Paris found against Lagarde over her handling of a dispute between the state and flamboyant businessman Bernard Tapie, which ended in a 404-million-euro (USD 422 million) award for Tapie.
Crucially, however, the Paris court exempted her from any penalty, citing her "international reputation" and the fact that at the time of the events in 2008 she had been busy fighting a global financial inferno.
The high-flying 60-year-old former corporate lawyer became the first female IMF chief in 2011, succeeding her disgraced compatriot Dominique Strauss-Kahn.
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She was not in the Paris court for the ruling. Her lawyer Patrick Maisonneuve told reporters she was in Washington for "professional reasons".
The French government expressed continued support for Lagarde, saying it "retained its utmost confidence in her capacity to carry out her responsibilities".
Lagarde was tried over her decision to allow the long- running dispute over Tapie's sale of Adidas to be resolved by a private arbitration panel instead of by the courts.
The court cleared her of negligence over that decision but upheld the charge over her failure to contest the staggering amount of the resulting award.
An investigation later showed the arbitration to be fraudulent.
Lagarde's case was heard by the Court of Justice of the Republic - a tribunal of judges and members of parliament that hears cases against ministers accused of wrongdoing in office.
The punishment for negligence theoretically carried a one-year prison sentence and a 15,000-euro (USD 15,676) fine.
Lagarde told the court during her five-day trial last week she had acted in good faith.
The court noted however that her inaction allowed Tapie and his wife to pocket 45 million euros in "moral damages" for alleged emotional harm - a sum included in the arbitration award.